French Drain Calculator
Enter your trench length, width, and depth to estimate the drainage gravel in cubic yards and tons and the length of perforated pipe needed for a French drain.
Results
- Drainage gravel
- 3.06 cu yd
- Estimated weight
- 4.28 tons
- Perforated pipe
- 50 ft
Estimates only. Round up and buy a little extra to account for waste, spills, and breakage.
A french drain is a gravel filled trench with a perforated pipe at the bottom. This calculator sizes the trench volume, the washed stone needed and the pipe and fabric run length.
Pipe displaces stone, so the calculator subtracts the pipe volume rather than leaving you with a surplus pile of gravel.
Also works as a drainage trench calculator, drain gravel calculator, and perforated pipe estimator.
How to use the french drain calculator
- Enter the trench length in ft.
- Enter the trench width in in.
- Enter the trench depth in in.
- Enter the waste factor in %.
- Optionally add a price to estimate the total project cost.
- Read your quantities instantly — no sign-up, and you can print the estimate to take to the supplier.
Worked example
Inputs
- Trench length
- 50 ft
- Trench width
- 12 in
- Trench depth
- 18 in
- Waste factor
- 10 %
Results
- Drainage gravel
- 3.06 cu yd
- Estimated weight
- 4.28 tons
- Perforated pipe
- 50 ft
Using the example values above, the french drain calculator returns 3.06 cu yd (drainage gravel). Change any field to match your own project and the numbers update instantly.
Trench sizing and slope
A typical residential french drain is 12 inches wide and 18 to 24 inches deep. Slope the trench at least 1 percent, which is 1 inch of fall per 8 feet of run.
Line the trench with non woven filter fabric, add 2 inches of stone, lay the perforated pipe holes down, then fill with washed 3/4 inch stone and fold the fabric over the top.
Stone choice matters
Use washed, angular stone with no fines. Crusher run or pea gravel with dust clogs the voids and the drain stops working within a few seasons.
A 50 ft trench at 12 inches wide and 18 inches deep needs about 2.8 cubic yards of stone, roughly 3.9 tons.
How much extra should I order?
Always order a little extra. A 5% waste factor suits simple, square projects; 10% is right for most jobs; bump it to 15–20% for irregular shapes, diagonal patterns, or uneven ground. Running short almost always costs more than the leftover material.
Formula
Gravel volume = length × width × depth. Cubic yards = volume ÷ 27. Tons = cubic yards × 1.4. Pipe length = trench length.
Frequently asked questions
- What gravel is best for a French drain?
- Use clean, washed 3/4-inch crushed stone or river rock so water flows freely and fines do not clog the drain.
- How deep should a French drain be?
- Most French drains are 12–24 inches deep and 6–12 inches wide, sloped at least 1% (1 inch per 10 feet) toward the outlet.
- How much gravel for a french drain?
- Multiply trench length by width by depth in feet and divide by 27, then subtract about 0.09 cubic feet per foot for 4 inch pipe.
- How deep should a french drain be?
- 18 to 24 inches for yard drainage and at least to the footing depth when draining water away from a foundation.
- Does a french drain need fabric?
- Yes. Non woven filter fabric keeps soil fines out of the stone and is the difference between a drain lasting 5 years and 30.
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